My copending U.S. application Ser. No. 07/914,489, whose teachings are incorporated herein by reference, discloses an ethylene oligomeriiation/trimerization process which uses a catalyst comprising a chromium complex which contains a coordinating polydentate ligand and an aluminoxane to produce high quality .alpha.-olefins which are enriched in 1-hexene. Suitable ligands include cyclic polyamines, and polypyrazolyl borates.
In my copending U.S. application Ser. No. 08/227,433 (which incorporates therein the disclosure of U.S. Ser. No. 08/025,524), I have shown that certain polydentate ligand complexes of chromium salts in combination with aluminoxanes can catalyze ethylene oligomerization, and especially ethylene trimerization to form 1-hexene with a very high degree of selectivity, e.g. about 96%, with formation of product containing less than 2 wt % of polyethylene. The disclosure of my copending U.S. application Ser. No. 08/227,433 is incorporated herein by reference.
Prior art ethylene trimerization processes, such as are described in U.S. Pat. Nos. 4,668,838 and 4,777,315 which use mixtures of a chromium compound, an aluminoxane and a third component selected from hydrocarbyl isonitriles, amines and ethers, are reported to produce amounts of polyethylene ranging from about 18 to 90+ percent as a coproduct. Such polyethylene not only decreases the yield of desirable product but also causes problems due to polymer build-up which would be expected to hamper the commercial use of such processes.